1. Field of the Disclosure
This disclosure relates generally to systems and methods for depositing a material on a substrate, such as a printed circuit board, and more particularly to an apparatus and a method for depositing viscous materials, such as solder paste, epoxies, underfill materials, encapsulants, and other assembly materials, on electronic substrates.
2. Discussion of Related Art
There are several types of prior art dispensing systems used for dispensing metered amounts of liquid or paste for a variety of applications. One such application is the assembly of integrated circuit chips and other electronic components onto circuit board substrates, which are sometimes referred to as electronic substrates. In this application, automated dispensing systems are used for dispensing dots of liquid epoxy or solder paste, or some other related material, onto circuit boards. Automated dispensing systems are also used for dispensing lines of underfill materials and encapsulants, which mechanically secure components to the circuit board. Underfill materials and encapsulants are used to improve the mechanical and environmental characteristics of the assembly.
Another application is to dispense very small amounts or dots onto a circuit board. In one system capable of dispensing dots of material, a dispenser unit utilizes a rotating auger having a helical groove to force material out of a nozzle and onto a circuit board. One such system is disclosed in U.S. Pat. No. 5,819,983, entitled LIQUID DISPENSING SYSTEM WITH SEALING AUGERING SCREW AND METHOD FOR DISPENSING, which is owned by Speedline Technologies, Inc. of Franklin, Mass., a subsidiary of the assignee of the present disclosure.
In an operation employing an auger-type dispenser, the dispenser unit is lowered towards the surface of the circuit board prior to dispensing a dot or a line of material onto the circuit board and raised after dispensing the dot or line of material. Using this type of dispenser, small, precise quantities of material may be placed with great accuracy. The time required to lower and raise the dispenser unit in a direction normal to the circuit board, typically known as a z-axis movement, can contribute to the time required to perform dispensing operations. Specifically, with auger-type dispensers, prior to dispensing the dot or line of material, the dispenser unit is lowered so that the material touches or “wets” the circuit board. The process of wetting contributes to additional time to perform the dispensing operation.
It is also known in the field of automated dispensers to launch or jet dots of viscous material toward the circuit board. In such a jetter-type system, a minute, discrete quantity of viscous material is ejected from a nozzle with sufficient inertia to enable the material to separate from the nozzle prior to contacting the circuit board. As discussed above, with the auger-type application or other prior, traditional dispensing systems, it is necessary to wet the circuit board with the dot of material prior to releasing the dot from the nozzle. When ejecting, the dots may be deposited on the substrate without wetting as a pattern of discrete dots, or alternatively the dots may be placed sufficiently close to each other to cause them to coalesce into more or less a continuous pattern. One such jetter-type system is disclosed in U.S. Pat. No. 7,980,197, entitled METHOD AND APPARATUS FOR DISPENSING A VISCOUS MATERIAL ON A SUBSTRATE, which is owned by Illinois Tool Works Inc. of Glenview, Illinois, the assignee of the present disclosure.
In the field of micro-jetting technology, ink jet printers traditionally use one or more ink jet heads for precisely projecting drops of ink onto a printing medium (such as paper) to generate text, graphical images or other indicia. Drops are projected from a minute external orifice in each head to the printing medium so as to form the text, graphical images or other indicia on the printing medium. A suitable control system synchronizes the generation of ink drops. It is important that the ink drops be of substantially uniform size, and also that the drops are applied consistently onto the printing medium so that printing is not distorted. It is also important, especially when depositing material onto an electronic substrate, that the drops are applied precisely onto the electronic substrate.
Existing ink jet technology, whether it is thermal-jet or piezo-jet, can only jet micro-droplets with low viscosity liquids (typically 2-15 centipoise), such as water based inks, and only for short printing distances. Thus, traditional ink jet systems are not particularly suited for dispensing the more viscous materials described above. In such existing ink jet technology, a pressure pulse is applied to a fluid chamber with sufficient pressure to overcome surface tension forces, thereby forming and ejecting a droplet of fluid from the ink jet nozzle. However, for jetting higher viscosity liquids (greater than 100 centipoise) with drop-on-demand requirement, there is no known ink jetting method.
In one basic type of ink jet head, ink drops are produced on demand, for example as disclosed in U.S. Pat. No. 4,106,032, entitled APPARATUS FOR APPLYING LIQUID DROPLETS TO A SURFACE BY USING A HIGH SPEED LAMINAR AIR FLOW TO ACCELERATE THE SAME, the entire disclosure of which is herein incorporated by reference. In such drop-on-demand ink jet heads, ink in an ink chamber in the ink jet head, in response to a pressure wave generated from an electric pulse applied to a piezoelectric crystal, flows through an ink passageway in an ink chamber wall and forms an ink drop at an internal drop-forming orifice outlet located at the outer surface of the ink chamber wall. The ink drop passes from the drop-forming orifice outlet, through an air chamber, and toward a main external orifice of the ink jet head leading to the print medium. Continuous air under pressure is delivered to the air chamber and propels the ink drop through the air chamber and to the print medium.
However, such prior art drop-on-demand ink jet heads suffer from numerous disadvantages, drawbacks and/or limitations, for example as discussed in U.S. Pat. No. 4,613,875, entitled AIR ASSISTED INK JET HEAD WITH PROJECTING INTERNAL INK DROP-FORMING ORIFICE OUTLET, and in U.S. Pat. No. 4,728,969, entitled AIR ASSISTED INK JET HEAD WITH SINGLE COMPARTMENT INK CHAMBER, the entire disclosures of these patents are herein incorporated by reference. In an attempt to improve upon such prior art drop-on-demand ink jet heads, an ink chamber with an ink drop-forming orifice outlet from which ink drops are generated in response to pressure waves caused by a piezoelectric crystal. This internal orifice outlet is centered in a projecting structure which extends toward an external orifice. The projecting structure is of a frustoconical or mesa-like shape. As stated therein, air flowing past the top (orifice outlet) of the projection prevents ink from wetting anything but the top of the projection, resulting in highly uniform ink drop formation with a single uniform dot being produced on the printing medium in response to each pressure wave.
The existing micro-jetting technology, whether it is piezo-jet, positive displacement or time pressure has inherent limitations on droplet size and fluid viscosity range for suitable jetted fluids, and therefore have not been successfully incorporated into dispensers and systems for applying such viscous materials onto electronic substrates.